On my scheduled lifting days i hit the bathroom for both varieties far more frequently, but I attribute it largely to the fact that my water intake is doubled on workout days to 2 gallons and my protein is upped to 350 grams or so...much of the extra through whey...other than that, i've not heard much about a workout itself of any kind inducing that type of reaction.

Obviously fiber will do it - it's not digestable and scrapes you clean. Also, if you drink any thermogenic drinks, they often contain a hefty dose of niacin which could also lead to a revisit to the throne.